This invention relates to the art of measuring parameters of blood within the cardiovascular system and, more particularly, to apparatus for directly measuring the velocity of blood flowing at a location of interest within a blood vessel by means of a catheter having a transducer tip insertable into the blood vessel.
It is known in the art to employ optically based catheters for use in measuring blood pressure within a cardiovascular system. Such devices typically employ a pressure transducer located at the distal end of a catheter which is insertable within a blood vessel for measuring blood pressure at a site of interest. Such devices typically take the form as illustrated in U.S. Patents to Polyanyi, U.S. Pat. No. 3,249,105 and Franke, U.S. Pat. No. 3,215,135. Each of these devices employs a catheter having fiber optic means extending the length of the catheter to the distal end thereof at which the fiber optic means is in optical communication with a pressure transducer. The pressure transducers in Polyanyi and Franke, supra, take the form of a diaphragm covering the end hole of a catheter. The diaphragm is located in front of or distal to the end of the optical fiber means and receives light and reflects it back into the fiber optic means for transmission to an externally located meter. Since the transducer is inserted into the bloodstream of a patient, the blood pressure deflects the diaphragm causing modulation of the light intensity so that the meter provides an indication of blood pressure.
During blood flow conditions, such catheters employing diaphragm covered end holes measure total pressure including both kinetic pressure and static pressure. That is, by aligning the end hole of a catheter with the direction of blood flow, kinetic energy terms are introduced. If the catheter end hole is directed upstream, a kinetic term will be added to the pressure, and, if the end hole is facing downstream, the kinetic term will be subtracted from the pressure. The magnitude of the error will vary with the velocity and density of the fluid. This error will vary during the course of a cardiac cycle and will distort the shape and magnitude of a pressure wave. In the pulmonary artery, the kinetic pressure may be on the order of 10% of total pressure at rest and 50% of total pressure at a cardiac output equal to three times that at rest. The importance of the kinetic pressure error is particularly great in stenotic areas where velocities are high.
Catheters are known, however, which may be employed for measuring static pressure rather than total pressure, as in the case of the catheters discussed above. Such static pressure measuring catheters may employ side port monitoring of pressure rather than end hole monitoring of pressure. Thus, when pressure is measured perpendicular to blood flow, the kinetic contribution is minimal. One catheter for measuring static pressure has been described in a 1978 article entitled "The Development of Fibre Optic Catheter Tip Pressure Transducer", Journal of Medical Engineering and Technology, Volume 2, No. 5, by H. Matsumoto and M. Saegusa. As described there, a side port membrane is responsive to pressure perpendicular to blood flow and causes movement of a mirror, which is mounted in cantilevered fashion to the membrane, within a cavity of the transducer. The mirror serves to reflect light received from fiber optic means extending the length of the catheter so that the intensity of light returned by way of the fiber optic means to a measuring device located outside the body is modulated in accordance with the static pressure. Another catheter employing side port monitoring of pressure is disclosed in my aforesaid U.S. application Ser. No. 671,913 filed Nov. 16, 1984 entitled "Improved Optical Fiber Pressure Transducer." The catheter disclosed there employs an optical fiber having its cladding removed for a portion of its length and replaced with a transducer constructed of flexible light absorbing material which circumscribes the unclad portion of the optical fiber. When inserted into a blood vessel, only pressure which is exerted perpendicular to the optical fiber causes the light absorbing material to be compressed against the core of the optical fiber causing an increase in the absoption of the light transmitted through the fiber core. This provides a measure of static blood pressure.
The present invention improves upon that disclosed in my aforesaid application by combining such static blood pressure monitoring with total blood pressure monitoring in such a manner that the static pressure components may be factored out leaving only a kinetic blood pressure component in the presence of blood flow. Such a kinetic blood pressure component, then, serves as a measure of blood velocity at the site of interest within a blood vessel. Such blood velocity measurements are useful as an estimate of cardiac output. Cardiac output values assist in the determination of the effectiveness of a ventricular contraction. Additionally, such blood velocity measurements may be employed to indicate stenotic regions in the blood vessels. Such stenotic regions may be attributable to arthromatous disease. Currently, these stenotic regions are typically visualized by injecting a radiopaque dye and visualizing the flow of the dye through the blood vessel. The degree of narrowing or stenosis is expressed as a percentage of the non-stenotic region. The measurement of pressure gradients through the stenotic region is another method. However, neither method is a sensitive indicator of either arthromatous disease progression or vessel functionality. Moreover, in a stenotic area of a blood vessel, such as in a mildly stenotic region, there is little change in pressure or area, but there is a discernable change in blood velocity. Consequently, by monitoring blood velocity at a site of interest, information can be obtained providing an early indication of stenosis and can be used to assess the success of a recanalization procedure.
In addition to measuring blood velocity at a site of interest, the present invention also contemplates simultaneously measuring static blood pressure at one or more sites of interest. It is known, however, in the prior art to provide a multiple site pressure transducer for providing simultaneous measurement of static blood pressure at a plurality of sites. One such example is disclosed in the U.S. patent to D. C. Brown, U.S. Pat. No. 4,543,961 assigned to the same assignee as the present invention. However, Brown proposes measurements of but a single blood parameter; namely, static blood pressure. There is no teaching of measuring other blood parameters, such as blood velocity.